"Of course I am fine."
Probably it marks wrong the fact of saying "watashi" but without the proper particle (I.e. "ga" if watashi is the topic, or "wa" if watashi is the subject).
And moreover, from what I've read in other threads, in Japanese it is preferred to avoid repating things implied on the context when possible. So, e.g. if someone made a question about you, you wouldn't need to specify the answer is about you. :)
In Japan they often leave out Watashi because it is usually assumed you are talking about yourself or whatever the last thing you mentioned. Ex. In English you would say ”i ate a taco. I then went to the gym. Afterwards i went to sleep" In Japanese they assume through the whole conversation your talking about you ”I ate a taco. Then went to the gym. Afterwards went to sleep" it can be a little odd in English but you will stand out if you constantly say "i" in Japan